There is known a valve device for an exhaust flow passage that changes a flow passage mode by opening and closing an exhaust flow passage in an exhaust system for an internal combustion engine. Patent Document 1, for example, discloses a valve device for an exhaust flow passage that opens and closes an opening arranged in a separator that partitions the inside of a muffler for an internal combustion engine into an upstream chamber and a downstream chamber. In this valve device for an exhaust flow passage, a valve body capable of closing the opening from a downstream side is supported by a support body in a rotationally movable manner, and the valve body is biased in a closing direction by a coil spring. Thus, in a low-revolution state in which the number of revolutions of the internal combustion engine is low, the pressure of exhaust gas in the upstream chamber is low, resulting in a valve closed state in which the opening is closed by the valve body. As a result, exhaust noise can be reduced. In contrast, in a high-revolution state in which the number of revolutions of the internal combustion engine has increased, the pressure of exhaust gas in the upstream chamber becomes high, which causes the valve body to be away from the opening, resulting in a valve open state in which the opening is open. As a result, pressure loss is reduced.
However, in the configuration described in Patent Document 1, as the valve body moves in an opening direction, the biasing force of the coil spring becomes larger. Thus, if the coil spring is designed to provide a larger biasing force with an emphasis on an effect of reducing exhaust noise in the low-revolution state, the effect of reducing pressure loss in the high-revolution state may be impaired. In contrast, if the coil spring is designed to provide a smaller biasing force with an emphasis on an effect of reducing pressure loss in the high-revolution state, the effect of reducing exhaust noise in the low-revolution state may be impaired.
To cope with this, a valve device for an exhaust flow passage comprising a link-type toggle mechanism has been proposed, as described in Patent Document 2, for example. The valve device for an exhaust flow passage of this kind comprises a first link member supported by a support body in a rotationally movable manner, and a second link member that is supported by a valve body in a rotationally movable manner and that is coupled to the first link member so as to be rotationally movable with respect to each other. That is, the support body, the valve body, the first link member, and the second link member form the toggle mechanism. Thus, in a valve closed state, the larger (i.e., the closer to 180 degrees) a link angle formed by the first link member and the second link member is, the stronger external force is required to rotationally move the valve body in an opening direction. Accordingly, as compared with the configuration in which the valve body is maintained in the valve closed state only with the biasing force of the biasing member (for example, the aforementioned configuration described in Patent Document 1), the external force required to open the valve body can be made larger, while reducing the biasing force of the biasing member.